Generally, a coating film is formed on the surface of a steel material such as a steel sheet to protect the surface thereof, prevent corrosion, and provide a sense of appeal to the material.
As a representative method of forming a coating film on a steel material such as a steel sheet, there is a pre-coated metal (PCM) method for directly coating a surface of a steel material with a resin composition such as paint, and a laminating method for adhering a film sheet on which a coating film is previously formed to a steel material.
However, in the conventional methods described above, since the resin composition for forming a coating film, coating paint, is prepared as a polyester or polyurethane type, which is coated on a steel material such as a steel sheet by a thermocuring method, high temperature drying equipment for evaporating a solvent during coating is needed.
Therefore, the resin composition described above requires more time for painting, resulting in a decrease in productivity.
Particularly, all conventional methods for forming a coating film on a surface of a metal material such as a steel product, for example, solvent evaporation-type painting, thermocurable painting, electrostatic and electrodeposition painting, and powder painting, need a heating process, and therefore production time and energy consumption is increased.
Furthermore, since conventional oil-type resin compositions, that is, paint, include materials causing air pollution and global warming, there is a need to develop more environmentally friendly methods.
For this reason, a method using a UV curable resin composition enabling environmental friendliness and mass production has been developed to resolve the above-mentioned problems. In Korea, a UV curable paint for paper was developed in the early 1980s, and various paint for wood and plastic are being developed and used. However, due to limitations of the UV curable paint, for example, failure in an adhesive strength and processibility caused by fast curing and radical polymerization, UV curable paints for metals have only developed occasionally and have limits in technical development.
Particularly, the UV curable painting method mentioned above is not suitable for coating a metal material such as a steel product because of high curing density and low adhesion.
As an example of such a UV curable resin composition, Korean Patent Publication No. 2006-0072475 discloses a UV curable paint composition for metal including 20 to 50 wt % of an epoxy acrylate oligomer, 10 to 40 wt % of a urethane acrylate oligomer, 20 to 40 wt % of a reactive acryl monomer, 5 to 10 wt % of a photoinitiator, and 1 to 5 wt % of an additive. In Korean Patent No. 0725249, as a UV curable paint for metal having excellent adhesion and anti-corrosion with respect to a metal material, a paint including 25 to 40 wt % of an epoxy or urethane acrylate oligomer, 15 to 25 wt % of a reactive acryl monomer, 20 to 30 wt % of a pigment, 5 to 10 wt % of a photoinitiator, 0.5 to 5 wt % of an adhesion promoter, and 3 to 5 wt % of an additive is disclosed.
However, since the above-mentioned conventional technique uses an inorganic anti-corrosive pigment to enhance corrosion resistance, the method cannot provide transparency to a steel product, and therefore the steel product has poor anti-corrosivity for preventing corrosion as well as poor bendability.